Defibrillators are implanted in patients susceptible to cardiac arrhythmias or fibrillation. Such devices provide cardioversion or defibrillation by delivering a high voltage shock to the patient's heart, typically about 500-750V. High voltage capacitors are used in defibrillators to accumulate the high voltage charge following detection of a tachyarrhythmia. In the effort to make implantable devices as small and thin as possible, flat aluminum electrolytic capacitors are used.
Such a flat capacitor is disclosed in U.S. Pat. No. 5,131,388 to Pless et al., which is incorporated herein by reference. Flat capacitors include a plurality of aluminum layers laminarly arranged in a stack. Each layer includes an anode and a cathode, with the anodes and cathodes being commonly connected to respective connectors. The layers may be cut in nearly any shape, to fit within a similarly shaped aluminum housing designed for a particular application. Normally, the cathode layers are together connected to the housing, while the anodes are together connected to a feed-through post that tightly passes through a hole on the housing, but which is electrically insulated from the housing. The feed-through post serves as an external connector for interfacing with other components.
Existing capacitors, such as illustrated in FIG. 1, and disclosed in U.S. patent application Ser. No. 08/876,274, filed Jun. 16, 1997, entitled "Aluminum Electrolytic Capacitor for Implantable Medical Device," now U.S. Pat. No. 5,926,357, which is hereby incorporated by reference, have addressed a trade off in the selection of feed-through 10 materials by using a readily solderable beryllium-copper post 12 passing through an insulated sleeve 14 in the housing 16, and covered within the housing by an aluminum cap 20 to which anode tabs 22 may readily be bonded or welded. A nut 24 is secured externally over the threaded copper post, to draw the cap to compress against an elastomeric gasket 26. This provides an environmental seal that contains electrolyte fluid within the housing, and which prevents the corrosive electrolyte from contacting and attacking the copper post.
While effective, the advantages of such existing capacitor feed-throughs are achieved at a relatively high manufacturing cost, due to their complexity. In addition, part tolerances must be tight, to avoid a condition in which an overly long sleeve engages the post head before the gasket has been adequately compressed. Further, imprecise manufacturing of the cap/post assembly can generate cracks in the cap that may admit corrosive electrolytic fluid to damage the copper post. Also, existing feed-throughs require a foil strip 30 attached to the anode tabs for connection to the feed-through cap, a difficult and time consuming process requiring skillful and precise alignment of the components to be welded. In addition, when more than one capacitor is used in a single device, external jumpers are needed to connect the capacitors together, typically in series. Such jumpers add to the number of parts and to the complexity of manufacturing.
In the prior art device shown in FIG. 1, the nut and adjacent case are sealed with an insulating resin (not shown) to cover the nut 24 and nearby portions of the housing. This normally prevents any arcing between the cathode-connected case and anode connected-nut, which are separated by only a very small gap 32 established by the thickness and radius of the sleeve flange, typically about 0.005-0.010 inch. While arcing is prevented by proper insulation, a minor failure of the insulative coating may generate a serious device failure. Although coating flaws may be detected by careful inspection, a more cost effective method of is desirable. Increasing the sleeve flange thickness would increase the gap, but at a cost of device size, as the post would need to be correspondingly lengthened to provide an adequate free portion for soldering.